Add Mail Contacts to Office 365 Exchange

In order to add external recipients to Distribution Lists, they need to be added as Mail Contacts in Microsoft Exchange.

1. Log in to the Exchange Admin Center at https://outlook.office365.com/ecp/
2. Under recipients click on contacts
3. In the upper right of the list of contacts click on the ➕ and then Mail Contact
4. Fill out the mail contact's details
5. The only required fields are
   1. Display Name - the name that shows up in the contact list
   2. Alias - just enter the email address here and replace the @ with ?
   3. External email address - the email address of the mail contact
6. Click Save